Apple is not the only company to use the "i" prefix. There are thousads of websites and products that use is.
I personally have always thought of the "i" as representing "internet". It gives a more techy / online feel to your product or service.
Of course, there are some products and websites that use "i" as the pronoun "I", making a sentence. Example: "iFly.com" or "iFish" imply "I Fly." and "I Fish."